Modeling stream heat budget with HFLUX in a subtropical alpine intermittent river

Surface water and groundwater interactions between the stream and the hyporheic zone profoundly affect river intermittency and biogeochemical processes, yet they are rarely quantified. As an excellent natural tracer, temperature was used to quantify unknown patterns of hydrologic fluxes and to understand their impact on heat budget over time and space. The Yousheng Creek (a first-order upstream of Chichiawan Creek in Taiwan) is one of the crucial habitats for the endangered species of Formosan land-locked salmon. In recent years, stream disconnection seriously limited the expansion of the salmon habitat, hampering the rehabilitation work. This study takes heat as a tracer to examine exchange processes in the intermittent reach of Yousheng Creek, with the application of fiber-optic distributed temperature sensor (FO-DTS). The combined use of FO-DTS, stream heat budget model (the HFLUX computer program), drone imagery, meteorological measurements, and field surveys allowed for identifying, quantifying, and mapping groundwater inputs beneath the 1924 meters reach. Analysis of the temperature traces measured from June 23rd to June 25th, 2019, have identified several active hyporheic zones and groundwater discharge points, providing significant cooling in the study section. HFLUX successfully modeled the river temperature through time and space with a normalized root mean square error of 3.12%. Inference from the model indicates a series of high infiltration zones at midstream whereas primary groundwater discharge from the downstream. The results suggest that different groundwater contributions along the Yousheng Creek significantly impact river temperatures. These insights of groundwater-surface water interactions can be applied to improve the knowledge of hydrology processes and energy budgets in headwaters